STRUCTURAL EVOLUTION OF THE DEADMAN CREEK THRUST: INTERACTIONS BETWEEN LARAMIDE CONTRACTION AND EARLY RIO GRANDE RIFT EXTENSION AND MAGMATISM IN THE SANGRE DE CRISTO RANGE, SOUTHERN COLORADO

The western flank of the Sangre de Cristo Range in southern Colorado contains exposures of Proterozoic gneisses and granitoids that are locally thrust over slivers of Mississippian, Devonian, and Ordovician sedimentary rocks (MDO) and intruded by Oligocene gabbro stocks and aplitic dikes. These rocks record a complex multiphase geologic history that includes folding and faulting related to Laramide contraction and normal faulting and magmatism associated with Rio Grande rift extension. We investigated this multistage deformation with detailed geologic mapping and kinematic analysis of sheared rocks related to the Deadman Creek thrust – a top-NE directed thrust fault that juxtaposes Proterozoic crystalline rocks in the hanging wall and MDO in the footwall. This structure has been deformed by subsequent NE-vergent fault-propagation folds and back-thrusts, locally reactivated as an extensional shear zone, and later dissected by brittle-plastic normal faults. One of these normal faults is localized on the steep to moderately NE-dipping folded limb of the Deadman Creek thrust. In addition, the footwall of the Deadman Creek thrust hosts gently dipping mylonite zones with SW- and NE-plunging lineations and fabrics indicative of top-SW sense shear, opposite of the Laramide contractional regime. These zones are locally 10s of meters thick and are characterized by plastic deformation of quartz, brittle deformation of feldspar, and extensive chloritization, indicating greenschist facies conditions. Quartz EBSD data are consistent with top-SW shear as well as local coaxial strain, and c-axis opening angles indicate deformation temperatures ranging from ~300–500° C. The thickest mylonite zones are found proximal to locally strained range-front Oligocene gabbroic intrusions, rather than along the contact of the Deadman Creek thrust, suggesting these mylonites are associated with gabbro emplacement. One of the NE-dipping brittle-plastic normal faults that cuts the Deadman Creek thrust involves an aplite intrusion with a ~26 Ma zircon U-Pb date. We interpret the top-SW shear zones and the brittle-plastic normal faults to record the earliest stages of Rio Grande rift extension that was coeval with bimodal magmatism and preceded development of the range-bounding Sangre de Cristo normal fault system in the Miocene.